The production of metal particles by the polyol process is known from, e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the entire disclosure of which is expressly incorporated by reference herein. In the polyol process, a metal precursor is reduced at an elevated temperature by a polyol to afford the corresponding metal in the form of particles (usually in the micron and nanometer size range). A number of metal precursors and in particular, a number of transition metal precursors can be converted to metal particles by this process. In a typical procedures a solid metal precursor is dissolved in a polyol and the solution is heated until the reduction of the metal precursor is substantially complete. Thereafter, the formed particles are isolated by separating them from the liquid phase, e.g., by centrifugation.
A modification of this method is described in, e.g., P.-Y. Silvert et al., “Preparation of colloidal silver dispersions by the polyol process” Part 1—Synthesis and characterization, J. Mater. Chem., 1996, 6(4), 573-577; and Part 2—Mechanism of particle formation, J. Mater. Chem., 1997, 7(2), 293-299. According to the Silvert et al. articles, the entire disclosures of which are expressly incorporated by reference herein, the polyol process is carried out in the presence of a polymer, i.e., polyvinylpyrrolidone (PVP). In particular, the PVP is dissolved in the polyol and helps to control the size and the dispersity of the metal particles. In a typical experiment, about 10 g of PVP was dissolved at room temperature in 75 ml of ethylene glycol and 2.4 mmole (400 mg) of silver nitrate was added to this solution. The resultant suspension was stirred at room temperature until the silver nitrate had dissolved completely, whereafter the system was heated to 120° C. and the reaction was conducted at this temperature for several hours. After cooling and dilution with water, the reaction mixture afforded silver particles having a mean particle size of 21 nm with a standard deviation of 16%.
While the reported results are desirable for some metal precursors, other metal precursors are not readily reducible to form metallic particles. For example, Ni2+ has a reduction potential of about −0.25 V. It is difficult to form metallic nanoparticles from metal precursors having a reduction potential less than about 0.6 V. In addition, metal nanoparticles, such as Co, Ni, and Cu-containing metal nanoparticles, formed from metal precursors having low standard reduction potentials, are often easily oxidized during a production process.
Thus, the need exists for processes for forming nanoparticles without oxidation from metal precursors having a low reduction potential, e.g., less than about 0.60 V, less than about 0.40 V, less than about 0.20 V or less than about 0.0 V. Nanoparticles formed from such processes would be useful in a variety of applications. For example, electronic inks comprising such nanoparticles could be employed in the fabrication of electrically conductive features for use in electronics, displays, and other applications.